How to connect a round duct to a rectangular vent?

My crawlspace has high humidity and a musty odor. Since it is now warm and sunny outside, I want to vent it out.
The idea is to put a 8-inch centrifugal duct fan in the middle of the crawlspace and connect a flex duct from the fan's output to one of the crawlspace vents.
So if there are 10 crawlspace vents, 9 will be sucking air and one blowing air. If this creates a slight negative pressure in the crawlspace that would be even better; it would counter the tendency for crawlspace odor to migrate into the living are.
Question is how best to connect a 8-inch round flex duct to a rectangular crawlspace vent (about 16-inch by 7 inch inside dimension). I don't think there is a "round duct to crawlspace vent adapter".
Also, the crawlspace wall is concrete so it's not easy to attach anything to the wall.
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Well, 8" dia is has a 8x3.14 = 25 " circumference.
The rect vent has a 2x16x7 = 224" circumference, so you will have to fit the round vent *into* the rect vent as you probably already figured out.
How about squishing it in, packing the empty spots with crunched plastic bags and covering with duct tape. Might that work?
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On 04/21/2015 02:15 AM, CRNG wrote:

Better check your formula, sparky. Maybe 2*(16+7)
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= 46. You are correct. Thank you. Let that be a lesson to me about trying to think at 02:15 in the morning.
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wrote:

floor register on the end of a round heat pipe. Don't know if there is a "stock" one that will do the job but any good HVAC sheet metal shop should be able to cobble one up for you.
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snipped-for-privacy@snyder.on.ca wrote:

I suggest the OP get a piece of sheet metal cut that fits the hole with an 8" round hole cut in it . Got to the local HVAC supply and purchase a boot like they use to connect those ducts to the plenum on the furnace . They come with a double-sticky gasket that will hold it in place while you put a few # 6 x 3/4" self tapping screws in . The type of canned foam used around doors and windows doesn't expand much and remains flexible , use a bead of that around the perimeter of the sheet meal where it meets the masonry (from the inside) .
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Terry Coombs wrote:

Ignore the above , I missed the part where the hole is only 7" tall . Instead of the plenum boot you need a transition for a 6X12 floor or wall vent attached to that piece of sheet metal . I'm not sure if that size has an 8" inlet , might also need a reducer to 6" .
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snipped-for-privacy@snyder.on.ca wrote:

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You can get the proper size fittings fabricated at a sheet metal shop. Some HVAC installers have their own duct fabrication shop. Look in the yellow pages.
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On 4/20/2015 10:01 PM, Bob wrote:

I'd call some HVAC places near you, and see if any of them can get you a transition from their stock, or from a parts house.
Since 7 inch isn't that far from your 8 round, maybe you can flatten the round flex, and let it hang out the opening a foot or so.
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On Tuesday, April 21, 2015 at 7:05:33 AM UTC-4, Stormin Mormon wrote:

+1
Either that or just look for a fan that is made to go inside round ducts to begin with. Either the fan or the transition piece sizes available should be online.
Does he even need a duct though? I would think if you just mounted one of the fans made for attic gables in one vent, consistent with the prevailing winds, it would probably be effective enough at pulling air through it.
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If the ducts are made of metal, the connector should also be of metal, thus made by either: -- a skilled and competent metalworker, or -- an amateur willing to learn beforehand for this one job the necessary skills, that he will never use again.
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Yes, there most definitely is. Check around. Lots of ducts are round and most vents are rectangular and the adapter looks a lot like the drapery adapter of a standard vacuum hose.
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I think there is.
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On 4/20/2015 10:01 PM, Bob wrote:

Have you checked with an HVAC supplier? There are an amazing number of transition units out there. You might even find what you need at the local big-box store -- they don't carry the full range but they certainly have adapters to go from round duct to standard registers. Even if you do find something it may not last long if the environment is really humid since most of them are made from the thinnest sheet metal. As a last resort, any sheet metal shop can make what you describe from heavy galvanized which will last longer than the flimsy stuff.
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